1. Field of the Invention
The present invention relates to a luminance/chrominance signal separation apparatus for a composite video signal, and more particularly to a two-dimensional adaptation type luminance/chrominance signal separation apparatus for separating the luminance signal from the chrominance signal in accordance with a correlation to a vertical direction.
2. Description of the Related Art
The present invention can be applied in the same principle to various existing color television methods such as National Television System Committee (NTSC), Phase Alteration by Line (PAL), Sequential Couleur a Memoire (SECAM) and the like. However, for the sake of convenience, the present invention will be described with respect to the composite video signal of NTSC television method by way of example.
The composite video signal of NTSC method comprises a luminance signal, a chrominance signal and a synchronous signal. The chrominance signal is in turn realized in a shape of color difference signals (R-Y, B-Y) or a Quadrature Modulation of I/Q signals.
FIG. 1 is a drawing for illustrating a construction of a composite video signal in accordance with NTSC method.
As illustrated in FIG. 1, the luminance signal Y has a signal band of approximately 4.3 MHz, and the chrominance signals I and Q have bandwidths of 2.17 MHz and 1.17 MHz respectively centered around a chrominance subcarrier (fsc=3.58).
Assuming that the composite video signal is V(t), the luminance signal is Y(t) and the chrominance signal C(t), then the composite video signal can be obtained by the following formula [1]. EQU V(t)=Y(t)+C(t) Formula 1.
The chrominance signal C(t) can be obtained by the following formula [2]for the quadrature-modulated signals of I(t) and Q(t). EQU C(t)=I(t) cos (2 fsct)+Q(t) sin (2 fsct) Formula 2.
It will be noted that fsc is approximately 3.58 MHz, i.e., the carrier frequency of the chrominance signal.
According to NTSC method, a relationship of the following formula [3]can be formed between the carrier frequency (fsc) of the chrominance signal and a frequency of horizontal synchronous signal fh. EQU fsc=(455/2)/fh Formula 3.
A method using a filter has been disclosed as a way by which the luminance signal and chrominance signal can be simply separated from the composite video signal having a construction as illustrated in FIG. 1.
In other words, the luminance signal is separated by means of a Low-Pass Filter (LPF) and the chrominance signal is separated by a Band-Pass Filter (BPF) having a band near 3.58 MHz.
This kind of method, however, cannot produce a satisfactory picture due to a generation of cross-talk between the luminance and chrominance signals in bands beyond 2 MHz where the luminance signal and chrominance signal are mixed.
FIG. 2 is a drawing showing a luminance/chrominance separation apparatus utilizing a Line Comb Filter, i.e., a circuit for showing a method utilizing a chrominance signal phase difference between horizontal scanning lines.
In FIG. 2, assuming a composite video signal Vi(t) represents an i-th horizontal scanning line, then Vi(t) can be obtained by the following formula [4]. EQU Vi(t)=Yi(t)+Ci(t) Formula 4.
Thus, a composite picture signal Vi+1 (t) for the i+1th horizontal scanning line can be obtained by the following formula [5]. EQU Vi+1(t)=Yi+1 (t)-Ci+1 (t) Formula 5.
Accordingly, a chrominance signal C' can be obtained by delaying the horizontal scanning line as much as one line 1H (where; H denotes a period of the horizontal scanning line), subtracting a front and a rear scanning signal thereof and then by dividing by 11/2.
The chrominance signal C' thus obtained is caused to pass BPF to thereby obtain a final chrominance signal C. The luminance signal Y can be obtained simply by, subtracting the chrominance signal C from the composite video signal V(t) being input.
The apparatus for separating the luminance and chrominance signals by means of the Line Comb Filter (LCF) as illustrated in FIG. 2 can obtain an improved picture with no cross-talk between the luminance signal and chrominance signal in the case of the picture possesses a vertical correlation. However, in the case of a picture with no correlation or a picture with few correlations, a phenomenon called hanging dot is generated to thereby deteriorate the picture quality.
An adaptation type luminance and chrominance signal separation apparatus for improving the aforesaid drawback is illustrated in FIG. 3.
According to FIG. 3, a detecting circuit 32 detects a composite video signal V being input, a signal VH, 1H-delayed therefrom and a signal VHH, 2H-delayed therefrom, and obtains a chrominance signal 2C from V and VH, and obtains a chrominance signal 2C also from VH and VHH.
A selection control unit 34a of a selection circuit 34 outputs a selection signal according to a correlation among V, VH and VHH detected from the detecting means 32.
A multiplexer 34B of the selection circuit 34 selects one of the chrominance signals 2C according to an output of the selection control unit 34A, and thereby outputs the same.
A level converter 34c converts by 1/2 the chrominance signal output from the multiplexer 34a to thereby output the signal C'.
The chrominance signal C' obtained from the foregoing circuitry is caused to pass BPF to thereby obtain the chrominance signal C. If the chrominance signal C is subtracted from the composite video signal being input, the luminance signal Y can be obtained, which reduces the generation of the hanging dot by way of the selective output to thereby improve the picture quality.
Specifically, the method thus illustrated by FIG. 3 is mainly applied to a digital comb filter utilizing digital signal processing or a Charge Coupled Device (CCD) comb filter. The method is cheaper in cost when compared with a three-dimensional process (a process on a time axis in addition to a two-dimensional vertical and horizontal process) and can easily be realized, which makes the same adequate for two-dimensional Y/C separation.
FIG. 4 is a block diagram for illustrating another embodiment of a conventional adaptation type Y/C separation apparatus.
According to FIG. 4, the apparatus receives the composite video signal V being input and a signal-1H delayed thereof VH and signal 2H-delayed thereof VHH. A chrominance signal (BOTTOM- 1H), is obtained by dividing 1/2 the difference signal between V and VH and a chrominance signal (TOP- 1H) is obtained by dividing 1/2 the difference signal between VH and VHH.
Furthermore, a chrominance signal (BPF) can be obtained from VH by way of the Band-Pass Filter of chrominance signal band, which is added by the chrominance signals (BOTTOM-1H and TOP-1H), and is divided by 1/2 to thereby obtain a chrominance signal (2H-COMB).
The selection control unit outputs the selection signal according to the correlations between V and VH, and between VH and VHH.
The multiplexer selects one of the chrominance signals C' (BPF, BOTTOM-1H, 2H-COMB or TOP-1H) to thereby output the same in accordance with the output of the selection control unit.
The chrominance signal C' selectively output from the multiplexer becomes a final chrominance signal C after passing through the Band-Pass Filter.
Meanwhile, the luminance signal Y can be obtained by subtracting the chrominance signal C from the signal, where VH is delayedly matched.
FIGS. 12A and 12B are drawings of two-dimensional region for illustrating operational states produced by the circuitry shown in FIG. 4.
According to FIG. 12A, correlations between V and VH, and between VH and VHH are obtained from the selection control unit of FIG. 4 whereby a lesser correlation is designated as MIN while a greater is designated as MAX.
Then, the region from which selection can be made from MIN and MAX exists on the upper side with a diagonal line as a center.
When respective reference values Ref1 and Ref2 are determined based on the foregoing against the MIN and MAX, and if the MIN and MAX values are smaller than the respective reference values Ref1 and Ref2, a region (2H-COMB) is selected.
In other words, if the difference between the upper and lower lines is less than a predetermined value, the selection control unit outputs a selection signal, so that the multiplexer can output the chrominance signal (2H-COMB) calculated from the V, VH and VHH.
Furthermore, if MIN is smaller than the reference value Ref1 while MAX is greater than the reference value Ref2, B region (1H-COMB) is selected so that the multiplexer can select a chrominance signal with a larger correlation among the chrominance signals (1H-COMB) to thereafter output the same.
If both MIN and MAX are larger than both references Ref1 and Ref2, which denotes that there is no correlation between the upper line and the lower line, the multiplexer is caused to select C region.
In other words, the chrominance signal BPF output from the Band-Pass Filter (BPF) or the chrominance signal (2H-COMB) calculated from V, VH and VHH is caused to be output in accordance with the selection signal randomly set by a user from the outside.
FIG. 12B is a drawing illustrating a set-up of reference values Ref1 and Ref2 in a different way from that shown in FIG. 12A wherefrom the chrominance signal BPF or 2H-COMB is output according to a selection signal randomly set by the user from the outside when MIN is greater than the reference value Ref1.
If MIN is less than the reference value Ref1, the chrominance signal 2H-COMB or 1H-COMB is selected according to MAX and the reference value Ref2, thereby outputting the same.
However, if, as described above, a selection of output is forcibly realized despite a minute difference against a value near the established values Ref1 and Ref2, the separated image becomes unnatural due to its sensitivity to the established values.